Speech Intelligibility Performance Research Articles

Relation between sound absorption and speech intelligibility in room, part 1: Theoretical investigation

In Japan, requirements and design guidelines on sound absorption of a variety of ordinary spaces are currently under consideration. In general, sound absorption works for reverberation suppression and noise reduction, both of which contributes to improve speech intelligibility performance. However, its combined effect is not fully clarified, especially on the relationship among of reverberation time, average absorption coefficient and speech intelligibility indices. In this paper, two theoretical case studies are performed based on the formulations of the clarity C50 and the useful-to-detrimental ratio U50 in the conditions without and with background noise. For rectangular rooms with an aspect ratio of 1:2:4, those indices are calculated at three source-to-receiver distances, changing the room volume widely under different noise levels. The results demonstrated that the absorption coefficient required for a target C50 increases with increasing the room volume, but it decreases beyond a certain volume. Moreover, it was shown that under high background noise, U50 is not effectively improved by increasing the absorption coefficient if the communication distance is much longer than the critical distance.

The value of headphone accommodations in Apple Airpods Pro for managing speech-in-noise hearing difficulties of individuals with normal audiograms

Objective To investigate the extent to which Headphone Accommodations in Apple AirPods Pro attend to the hearing needs of individuals with normal audiograms who experience hearing difficulties in noisy environments. Design Single-arm interventional study using acoustic measures, speech-in-noise laboratory testing, and real-world measures via questionnaires and ecological momentary assessment. Study sample Seventeen normal-hearing individuals (9 female, 21–59 years) with self-reported hearing-in-noise difficulties. Results Acoustic measures showed that, relative to unaided, AirPods Pro provided a SNR advantage of +5.4 dB. Speech intelligibility performance in laboratory testing increased 11.8% with AirPods Pro, relative to unaided. On average, participants trialling AirPods Pro in real-world noisy venues reported that their overall hearing experience was a bit better than without them. Five participants (29%) reported that they would continue using AirPods Pro in the future. The most relevant barriers that would discourage their future use were limited hearing benefit, discomfort, and stigma. Conclusions Occasional use of AirPods Pro may help some individuals with normal audiograms ameliorate their speech-in-noise hearing difficulties. The identified barriers may inspire the development of new technological solutions aimed at providing an optimal management strategy for the hearing difficulties of this segment of the population.

THE ASSESSMENT OF ACOUSTICAL PERFORMANCE OF MOSQUES IN SHAH ALAM

A mosque is a place where Muslims conduct activities such as praying and listening to talks. Therefore, it is important for a mosque to have great acoustics quality as it influences the worshipper’s experience when praying or listening to Imam’s preaches. The acoustical evaluation in the mosque is rarely conducted and, in most cases, the development of good acoustic quality in the mosque is never considered at an early stage of design. In the present investigation, two large-volume mosques in Shah Alam were investigated: Section 7 Mosque (MS7) and Sultan Salahuddin Abdul Aziz Mosque (MSA). The objective of this study is to evaluate the acoustical characteristics of the mosque and its quality level. In addition, the speech intelligibility performance of each mosque was also determined using Speech Transmission Index (STI) and Rapid Transmission Index (RASTI). The experimental work found that the reverberation time of MSA has attained recommended values of reverberation time for that given size and volume while MS7 has slightly higher. The speech level attainment of both mosques is sufficient for worshippers to hear comfortably, but the background noise is high. Nevertheless, the signal-to-noise ratio (S/N ratio) indicates that both mosques have been able to obtain more than 15 dB(A) for improved speech intelligibility. The STI and RASTI prediction shows that MS7 has attained ‘fair’ and ‘bad’ ratings. In contrast, ‘poor’ and ‘bad’ ratings are achieved in the MSA.

The effect of explicit and implicit voice training on speech-on-speech intelligibility and listening effort

Listening to familiar voices might improve intelligibility of target speech in multiple-talker situations. In addition to personally familiar voices, voice training—through implicit exposure or explicit learning of a previously unheard voice—can improve speech intelligibility. However, there is no consensus on which method is more effective. We investigated the effect of explicit and implicit voice training on speech-on-speech perception and listening effort (pupillometry), among normal hearing listeners. There was no significant difference in speech intelligibility performance between trained and untrained voices, with either voice training method. However, an interaction between voice training method and target-to-masker ratio (TMR) on accuracy scores showed that the benefit of increased TMR on accuracy scores was significantly larger when voice training was implicit compared to explicit. Pupillometry results showed that explicit voice training significantly reduced listening effort for most disadvantageous TMR (−6dB), but not for more advantageous TMRs (0 and + 6 dB), and not for implicitly trained voices. Our results may imply that explicit voice training is more effective than implicit voice training in reducing listening effort. Still, implicit voice training, perhaps via procedural learning, might be more beneficial on the performance level when TMR becomes more advantageous.

The impact of formal musical training on speech intelligibility performance: Implications for music pedagogy in high-consequence industries

In high-risk industries, responding accurately and promptly to spoken commands is crucial to ensure safety and productivity. When simultaneous sounds occur, it increases the difficulty of responding efficiently and accurately. This study examined responses to spoken commands in an audio-visual multitasking paradigm that simulated a high-consequence working environment. Participants had varying levels of formal music training (FMT): no FMT, 1–3, 3–5, and 5+ years of FMT. We found that FMT groups were similar in accuracy overall and background music decreased accuracy for all groups. The degree of FMT significantly impacted non-response. Additionally, there was an interaction where the no FMT group responded 1.5-fold less frequently in the presence of background music; however, participants with 5+ years of FMT had no decrease. Considering the similar accuracy with less frequent responding, the 5+ years FMT group appears well calibrated to when they were unsure of the answer, suggestive of improved meta-cognition which could be valuable in high-risk industry settings. Future research can examine performance within different types or quality of musical training, and pedagogical approaches can be compared.

Improving the Efficiency of Dysarthria Voice Conversion System Based on Data Augmentation.

Dysarthria, a speech disorder often caused by neurological damage, compromises the control of vocal muscles in patients, making their speech unclear and communication troublesome. Recently, voice-driven methods have been proposed to improve the speech intelligibility of patients with dysarthria. However, most methods require a significant representation of both the patient's and target speaker's corpus, which is problematic. This study aims to propose a data augmentation-based voice conversion (VC) system to reduce the recording burden on the speaker. We propose dysarthria voice conversion 3.1 (DVC 3.1) based on a data augmentation approach, including text-to-speech and StarGAN-VC architecture, to synthesize a large target and patient-like corpus to lower the burden of recording. An objective evaluation metric of the Google automatic speech recognition (Google ASR) system and a listening test were used to demonstrate the speech intelligibility benefits of DVC 3.1 under free-talk conditions. The DVC system without data augmentation (DVC 3.0) was used for comparison. Subjective and objective evaluation based on the experimental results indicated that the proposed DVC 3.1 system enhanced the Google ASR of two dysarthria patients by approximately [62.4%, 43.3%] and [55.9%, 57.3%] compared to unprocessed dysarthria speech and the DVC 3.0 system, respectively. Further, the proposed DVC 3.1 increased the speech intelligibility of two dysarthria patients by approximately [54.2%, 22.3%] and [63.4%, 70.1%] compared to unprocessed dysarthria speech and the DVC 3.0 system, respectively. The proposed DVC 3.1 system offers significant potential to improve the speech intelligibility performance of patients with dysarthria and enhance verbal communication quality.

The impact of formal musical training on speech intelligibility performance—Implications for music pedagogy in high-consequence industries

In high-risk domains, accurate and timely communications while multitasking are necessary for safety and efficiency. Complex musical/acoustic environments cause hindered communication and awareness. This study used an audio-visual multi-tasking paradigm that examined speech intelligibility and if formal music training (FMT) can mitigate these challenges. Twenty-five students (16F/9M) from Vanderbilt University participated with varying levels of FMT: no FMT, 1–3 years, 3–5 years, and 5+ years of FMT. The study found that the degree of FMT significantly impacted non-response (p-value < 0.001). Among participants with no FMT, the presence of background music increased the odds of non-response by 1.5-fold (95% CI: 0.95, 2.37), conversely, participants with 5+ years of FMT had no decrease (OR: 0.97, 95% CI: 0.69, 1.36), showing that non-response in the presence of music worsens with each subgroup until 5+ years of FMT. The accuracy for all groups was similar (p = 0.74) and the background music decreased accuracy for all groups (OR: 0.67, 95% CI: 0.58, 0.76). Although levels of accuracy were similar for all, the 5+ FMT group responded less frequently, which may indicate increased working memory (i.e., phonological loop) and meta-cognition, a valuable skill in high-risk industry. Future research can explore the pedagogy of musical training.

Phonetic posteriorgram-based voice conversion system to improve speech intelligibility of dysarthric patients

Background and ObjectiveMost dysarthric patients encounter communication problems due to unintelligible speech. Currently, there are many voice-driven systems aimed at improving their speech intelligibility; however, the intelligibility performance of these systems are affected by challenging application conditions (e.g., time variance of patient's speech and background noise). To alleviate these problems, we proposed a dysarthria voice conversion (DVC) system for dysarthric patients and investigated the benefits under challenging application conditions. MethodA deep learning-based voice conversion system with phonetic posteriorgram (PPG) features, called the DVC-PPG system, was proposed in this study. An objective-evaluation metric of Google automatic speech recognition (Google ASR) system and a listening test were used to demonstrate the speech intelligibility benefits of DVC-PPG under quiet and noisy test conditions; besides, the well-known voice conversion system using mel-spectrogram, DVC-Mels, was used for comparison to verify the benefits of the proposed DVC-PPG system. ResultsThe objective-evaluation metric of Google ASR showed the average accuracy of two subjects in the duplicate and outside test conditions while the DVC-PPG system provided higher speech recognitions rate (83.2% and 67.5%) than dysarthric speech (36.5% and 26.9%) and DVC-Mels (52.9% and 33.8%) under quiet conditions. However, the DVC-PPG system provided more stable performance than the DVC-Mels under noisy test conditions. In addition, the results of the listening test showed that the speech-intelligibility performance of DVC-PPG was better than those obtained via the dysarthria speech and DVC-Mels under the duplicate and outside conditions, respectively. ConclusionsThe objective-evaluation metric and listening test results showed that the recognition rate of the proposed DVC-PPG system was significantly higher than those obtained via the original dysarthric speech and DVC-Mels system. Therefore, it can be inferred from our study that the DVC-PPG system can improve the ability of dysarthric patients to communicate with people under challenging application conditions.

Hyperbilirobinemia and Its Effect on Auditory Perception and Speech Intelligibility in Cochlear Implanted Children (Cochlear Implantation Outcomes in Children Who Have Experienced Hyperbilirobinemia).

The goal of this study was to investigate the probable difference in auditory perception and speech intelligibility performance amongst cochlear implanted children who experienced hyperbilirubinemia or auditory neuropathy in comparison to the cochlear implanted children with unknown etiology for hearing loss. This case-control study was carried out on 106 cochlear implanted children with mean age of 32.36 ± 11.98months who were purposively selected and allocated into four groups. Out of the total, 30 had no specific etiology for hearing loss, while the others had experienced auditory neuropathy or hyperbilirubinemia with/without blood exchange. The auditory perception and speech intelligibility performance of all the participants who had received auditory verbal therapy were assessed after 6 and 12 months of rehabilitation. Then, the data was analyzed, using the Statistical Package for Social Sciences-version 21(SPSS-21). Results indicated poor auditory perception and speech intelligibility performance of the cochlear implanted children with hyperbilirubinemia and blood exchange (P ≤ 0.05), while the participants in the control group with no specific etiology for hearing loss, the children with hyperbilirubinemia with no blood exchange, and those who suffered from auditory neuropathy performed better, respectively. Also, a significant correlation between auditory neuropathy and hyperbilirubinemia was observed. Despite lower improvement of auditory perception and speech intelligibility of the hearing impaired children who were experiencing moderate to severe degrees of hyperbilirubinemia or auditory neuropathy, cochlear implantation is highly recommended not only for children with unknown etiology for severe hearing loss but also for this group of hearing impaired children.

Objective Assessment of Speech Intelligibility in Crowded Public Spaces.

The objective of this study was to obtain a normative database of speech intelligibility data for young normal-hearing listeners communicating in public spaces. A total of 174 listeners participated in an interactive speech intelligibility task that required four-person groups to conduct a live version of the Modified Rhyme Test in noisy public spaces. The public spaces tested included a college library, a college cafeteria, a casual dining restaurant during lunch hour, and a crowded bar during happy hour. At the start of each trial, one of the participants was randomly selected as the talker, and a tablet computer was used to prompt them to say a word aloud from the Modified Rhyme Test. Then, the other three participants were required to select this word from one of six rhyming alternatives displayed on three other tablet computers. The tablet computers were also used to record the SPL at each listener location during and after the interval where the target talker was speaking. These SPL measurements were used to estimate the signal-to-noise ratio (SNR) in each trial of the experiment. As expected, the results show that speech intelligibility decreases, response time increases, and perceived difficulty increases as the background noise level increases. There was also a systematic decrease in SNR with increasing background noise, with SNR decreasing 0.44 dB for every 1 dB increase in ambient noise level above 60 dB. Overall, the results of this study have demonstrated how low-cost tablet computer-based data collection systems can be used to collect live-talker speech intelligibility data in real-world environments. We believe these techniques could be adapted for use in future studies focused on obtaining ecologically valid assessments of the effects of age, hearing impairment, amplification, and other factors on speech intelligibility performance in real-world environments.

A joint-feature learning-based voice conversion system for dysarthric user based on deep learning technology.

Dysarthria speakers suffer from poor communication, and voice conversion (VC) technology is a potential approach for improving their speech quality. This study presents a joint feature learning approach to improve a sub-band deep neural network-based VC system, termed J_SBDNN. In this study, a listening test of speech intelligibility is used to confirm the benefits of the proposed J_SBDNN VC system, with several well-known VC approaches being used for comparison. The results showed that the J_SBDNN VC system provided a higher speech intelligibility performance than other VC approaches in most test conditions. It implies that the J_SBDNN VC system could potentially be used as one of the electronic assistive technologies to improve the speech quality for a dysarthric speaker.

Benefits of a contralateral routing of signal device for unilateral Naída CI cochlear implant recipients.

Many bilaterally deaf adults are only able to receive one cochlear implant (CI), resulting in suboptimal listening performance, especially in challenging listening environments. Adding a contralateral routing of signal (CROS) device to a unilateral CI is one possibility to alleviate these challenges. This study examined the benefit of such a CROS device. Thirteen adult subjects with at least 6months of CI use, and no or limited benefit of a hearing instrument in the contralateral ear were included in the study. The perceived benefit of a CROS device in everyday listening environments was evaluated up to 1 year after initial fitting using several questionnaires. Speech intelligibility performance was determined using the French matrix sentence test in quiet and in two speech-in-noise setups and was followed for 3 months after CROS fitting. Subjects indicated high satisfaction with the practical usability of the CROS device and long-term device retention was high. Perceived benefits in everyday listening environments were reported. Formal speech intelligibility tests revealed statistically significant median improvements of 6.93dB SPL (Wilcoxon Z = 2.380, p = 0.017) in quiet and up to 8.00dB SNR (Wilcoxon Z = 2.366, p = 0.018) in noise. These benefits were accessible immediately without a need for prolonged acclimatization. Subjective satisfaction and device retention as well as speech intelligibility benefits in quiet and in noise prove the CROS device to be a valuable addition to a unilateral CI in cases of bilateral deafness where bilateral implantation is not an option.

Improving speech intelligibility performance for aircraft maintainers by selecting an appropriate combination of hearing protection devices

Aircraft maintainers work in hazardous noise environments throughout their career. Hazardous noise levels on the flight line vary, affecting the level of hearing protection required to reduce their risk of hearing loss and other hearing related disabilities. Current practice is to provide double hearing protection: a circumaural headset/earmuff with foam earplugs. This combination may provide an adequate amount of protection from aircraft noise, but the proper use of foam earplugs may degrade speech intelligibility (SI) performance. Due to this degradation, many maintainers fit the foam earplugs incorrectly in order to improve SI performance, therefore lowering the amount of protection provided. The objective of this study was to assess the noise attenuation and SI performance of a headset and filtered earplugs worn alone and in combination to provide a more appropriate recommendation of hearing protection devices. The filtered earplug can provide suitable levels of protection when worn alone or in combination with a headset (depending on the noise environment) and can provide adequate “hear through” capability for acceptable SI performance when communicating face-to-face (single hearing protection) or through the aircraft communication system (double hearing protection).

On the Relationship Between Short-Time Objective Intelligibility and Short-Time Spectral-Amplitude Mean-Square Error for Speech Enhancement

The majority of deep neural network (DNN) based speech enhancement algorithms rely on the mean-square error (MSE) criterion of short-time spectral amplitudes (STSA), which has no apparent link to human perception, e.g. speech intelligibility. Short-Time Objective Intelligibility (STOI), a popular state-of-the-art speech intelligibility estimator, on the other hand, relies on linear correlation of speech temporal envelopes. This raises the question if a DNN training criterion based on envelope linear correlation (ELC) can lead to improved speech intelligibility performance of DNN based speech enhancement algorithms compared to algorithms based on the STSA-MSE criterion. In this paper we derive that, under certain general conditions, the STSA-MSE and ELC criteria are practically equivalent, and we provide empirical data to support our theoretical results. Furthermore, our experimental findings suggest that the standard STSA minimum-MSE estimator is near optimal, if the objective is to enhance noisy speech in a manner which is optimal with respect to the STOI speech intelligibility estimator.

The Important Role of Contextual Information in Speech Perception in Cochlear Implant Users and Its Consequences in Speech Tests.

This study investigated the role of contextual information in speech intelligibility, the influence of verbal working memory on the use of contextual information, and the suitability of an ecologically valid sentence test containing contextual information, compared with a CNC (Consonant-Nucleus-Consonant) word test, in cochlear implant (CI) users. Speech intelligibility performance was assessed in 50 postlingual adult CI users on sentence lists and on CNC word lists. Results were compared with a normal-hearing (NH) group. The influence of contextual information was calculated from three different context models. Working memory capacity was measured with a Reading Span Test. CI recipients made significantly more use of contextual information in recognition of CNC words and sentences than NH listeners. Their use of contextual information in sentences was related to verbal working memory capacity but not to age, indicating that the ability to use context is dependent on cognitive abilities, regardless of age. The presence of context in sentences enhanced the sensitivity to differences in sensory bottom-up information but also increased the risk of a ceiling effect. A sentence test appeared to be suitable in CI users if word scoring is used and noise is added for the best performers.

Adaptive Wiener Gain to Improve Sound Quality on Nonnegative Matrix Factorization-Based Noise Reduction System

Nonnegative matrix factorization (NMF) is a useful decomposition technique for multivariate data. More recently, NMF technology was used as a noise-estimation stage for a Wiener-filtering-based noise reduction (NR) method to improve the quality of noisy speech. Previous studies showed that this method provides better sound quality performance than conventional NMF-based approaches; however, there is still scope for improving the performance under noisy listening conditions. More specifically, the performance of an NMF noise estimator for calculating the noise level is considered sensitive to diverse noise environments and signal-to-noise ratio conditions. Therefore, we proposed an adaptive algorithm that derives an adaptive factor ($\alpha $ ) to adjust the weight between the estimated speech and noise levels on the basis of the signal-to-noise level for the gain function of the Wiener-filtering-based NR method to further improve the sound quality. Two objective evaluations and listening tests evaluated the benefits of the proposed method, and experimental results show that better output sound quality and competed for speech intelligibility performance can be achieved when compared with conventional unsupervised NR and NMF-based methods.

Comparisons of the Korean Band-Importance and Absolute Transfer Functions according to Speakers’ Gender

Purpose The speech intelligibility index (SII) represents the contribution of audible speech information within given frequency bands to speech intelligibility. Generally, speech intelligibility performance depends on the gender of the speaker. The purpose of this study was to compare how a speaker’s gender affects band-importance functions (BIFs) and absolute transfer functions (ATFs), which are important components of the SII. Methods This study consisted of 78 subjects. The SII data for male speakers was obtained from a previous study. The SII data for female speakers was obtained using the same procedure as the previous study. Hearing-In-Noise Test sentences were used as stimuli. Intelligibility scores were measured in 42 filters at 5 signal-to-noise ratios. The BIFs and ATFs were derived using a nonlinear optimization procedure, and then the BIFs and ATFs of the different genders were compared. Results The BIF and ATF of different genders have different characteristics. In the case of the BIF, the gap between the different genders ranged between 0.2% (1,170 Hz) and 4.0% (8,500 Hz) depending on the frequency band. In the case of the ATF, the female speakers’ ATF was steeper than the male speakers’ ATF. Conclusion The results of the current study indicate that gender can be an important factor for predicting intelligibility. Female speakers’ speech seems particularly more intelligible than male speakers’ speech when measured at the same audibility levels in Korean SII modeling. Key Words: Speech intelligibility index, Gender effect, Speech perception, Band-importance function, Absolute transfer function 중심 단어: ì–´ìŒëª ë£Œì§€ìˆ˜, 성별 효과, 어음인지, 주파수중요함수, ì ˆëŒ€ì „ë‹¬í•¨ìˆ˜

Speech enhancement for cochlear implant recipients.

In this study, a single microphone speech enhancement algorithm is proposed to improve speech intelligibility for cochlear implant recipients. The proposed algorithm combines harmonic structure estimation with a subsequent statistical based speech enhancement stage. Traditional minimum mean square error (MMSE) based speech enhancement methods typically focus on statistical characteristics of the noise and track the noise variance along time dimension. The MMSE method is usually effective for stationary noise, but not as useful for non-stationary noise. To address both stationary and non-stationary noise, the current proposed method not only tracks noise over time, but also estimates the noise structure along the frequency dimension by exploiting the harmonic structure of the target speech. Next, the estimated noise is employed in the traditional MMSE framework for speech enhancement. To evaluate the proposed speech enhancement solution, a formal listener evaluation was performed with 6 cochlear implant recipients. The results suggest that a substantial improvement in speech intelligibility performance can be gained for cochlear implant recipients in noisy environments.

Assessment of the Speech Intelligibility Performance of Post Lingual Cochlear Implant Users at Different Signal-to-Noise Ratios Using the Turkish Matrix Test.

Spoken word recognition and speech perception tests in quiet are being used as a routine in assessment of the benefit which children and adult cochlear implant users receive from their devices. Cochlear implant users generally demonstrate high level performances in these test materials as they are able to achieve high level speech perception ability in quiet situations. Although these test materials provide valuable information regarding Cochlear Implant (CI) users' performances in optimal listening conditions, they do not give realistic information regarding performances in adverse listening conditions, which is the case in the everyday environment. The aim of this study was to assess the speech intelligibility performance of post lingual CI users in the presence of noise at different signal-to-noise ratio with the Matrix Test developed for Turkish language. Cross-sectional study. The thirty post lingual implant user adult subjects, who had been using implants for a minimum of one year, were evaluated with Turkish Matrix test. Subjects' speech intelligibility was measured using the adaptive and non-adaptive Matrix Test in quiet and noisy environments. The results of the study show a correlation between Pure Tone Average (PTA) values of the subjects and Matrix test Speech Reception Threshold (SRT) values in the quiet. Hence, it is possible to asses PTA values of CI users using the Matrix Test also. However, no correlations were found between Matrix SRT values in the quiet and Matrix SRT values in noise. Similarly, the correlation between PTA values and intelligibility scores in noise was also not significant. Therefore, it may not be possible to assess the intelligibility performance of CI users using test batteries performed in quiet conditions. The Matrix Test can be used to assess the benefit of CI users from their systems in everyday life, since it is possible to perform intelligibility test with the Matrix test using a material that CI users experience in their everyday life and it is possible to assess their difficulty in speech discrimination in noisy conditions they have to cope with.

Speech onset enhancement improves intelligibility in adverse listening conditions for cochlear implant users

Speech perception by cochlear implant (CI) users can be very good in quiet but their speech intelligibility (SI) performance decreases in noisy environments. Because recent studies have shown that transient parts of the speech envelope are most important for SI in normal-hearing (NH) listeners, the enhanced envelope (EE) strategy was developed to emphasize onset cues of the speech envelope in the CI signal processing chain. The influence of enhancement of the onsets of the speech envelope on SI was investigated with CI users for speech in stationary speech-shaped noise (SSN) and with an interfering talker. All CI users showed an immediate benefit when a priori knowledge was used for the onset enhancement. A SI improvement was obtained at signal-to-noise ratios (SNRs) below 6 dB, corresponding to a speech reception threshold (SRT) improvement of 2.1 dB. Furthermore, stop consonant reception was improved with the EE strategy in quiet and in SSN at 6 dB SNR. For speech in speech, the SRT improvements were 2.1 dB and 1 dB when the onsets of the target speaker with a priori knowledge of the signal components or of the mixture of the target and the interfering speaker were enhanced, respectively. The latter demonstrates that a small benefit can be obtained without a priori knowledge.